The rigidity of the machine tool refers to the ability to resist deformation under the action of cutting force and other forces. CNC machine tools have higher static stiffness and dynamic stiffness than ordinary machine tools. There is a standard that the stiffness coefficient of CNC machine tools should be 50% higher than similar ordinary machine tools.
In the process of cutting, the machine tool has to bear various external forces. The static force that it bears has the weight of the moving parts and the parts to be processed. The dynamic forces that are subjected to it are: cutting force, driving force, inertia force caused by acceleration and deceleration, friction Resistance and so on. Under the action of these forces, the structural components of the machine tool will be deformed, such as the contact deformation of the fixed joint surface or the moving meshing surface; the bending and torsional deformation of each support part, and the local deformation of some support members, etc., these deformations will be directly Or indirectly causing relative displacement between the tool and the workpiece, resulting in machining errors in the workpiece or affecting the characteristics of the machine cutting process.
Due to the complex transient conditions of the machining state, it is often difficult to make accurate theoretical calculations of the structural stiffness. The designer can only calculate the stiffness of some components (such as shafts, screws, etc.), and bend and torsion deformation of the bed, column, table and box parts, contact deformation of the joint surface, etc. It can be simplified and then approximated. The calculation result is often very different from the actual one, so it can only be used as a reference for qualitative analysis. In recent years, although the finite element method is used for analysis and calculation in machine tool structure design, in general, it is still necessary to test, analyze and compare models, objects or similar prototypes to determine reasonable structural solutions. By following the following principles and measures, the structural rigidity of the machine tool can still be reasonably improved.
1. Reasonable selection of the structural form of the component
(1) Correctly select the shape and size of the section
After the members are subjected to bending and torsional loads, the deformation depends on the bending and torsional moments of the section, and the bending and torsional moments of inertia are high. Table 7-1 lists the moments of inertia for each section shape with the same cross-sectional area (ie, the same gravity). From the data in the table, it can be known that when the same cross-sectional area is maintained, the wall thickness should be reduced and the contour size of the cross-section should be increased. The torsional stiffness of the circular cross-section is larger than that of the square cross-section, and the bending rigidity is It is smaller than the square section; the stiffness of the closed section is much larger than that of the unclosed section; the opening on the wall will reduce the stiffness, and the flange around the hole will restore the bending stiffness.
(2) Reasonable selection and arrangement of partitions and ribs
Reasonably arranging the partitions and ribs of the support member can improve the static and dynamic stiffness of the member. The structure of several columns shown in Figure 7-1 is arranged with longitudinal, transverse and diagonal ribs inside. The results of static and dynamic stiffness tests are listed in Table 7-2. Among them, the cross rib (No. 5) works best.
For some thin-walled members, in order to reduce the warpage of the wall surface and the distortion of the cross-section of the member, the ribs shown in Fig. 7-2 may be arranged on the wall plate, wherein the honeycomb reinforcing ribs are better, as shown in the figure (f) Shown. In addition to improving the stiffness of the component, it also reduces the shrinkage stress during casting.
(3) Improve the local stiffness of the component
The coupling parts of the guide rail and the support of the machine tool are often the weakest part of the local stiffness, but the connection mode has a great influence on the local stiffness. Figure 7-3 shows several forms of rail and bed coupling. If the rail is wider, apply a double wall connection, as shown in (d), (e), (f). When the guide rail is narrow, a single wall or a thick single wall joint may be used, or a vertical rib may be added to the single wall to improve the local stiffness.
(4) Components using welded structures
Supporting parts such as bed and column of the machine tool are welded by steel plate and section steel, which have the significant advantages of reducing the quality and increasing the rigidity. The modulus of elasticity of steel is about twice that of cast iron. If the shape and the contour are the same, if the rigidity of the welded part is the same as that of the casting, the wall thickness of the welded part is only half of the casting; if the local stiffness is required to be the same, Since the local stiffness is proportional to the cube of the wall thickness, the wall thickness of the welded part is only about 80% of the wall thickness of the casting. In addition, whether the stiffness is the same to reduce the mass, or the mass is the same to increase the stiffness, the resonant frequency of the member can be increased, making resonance less likely to occur. Welding with steel plates makes it possible to form the members into a fully enclosed box-shaped structure, which is advantageous for increasing the rigidity of the members.
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